Flow control valves, such as ball valves, are well known in the art and commonly comprise a valve body or housing having an interior cavity and a pair of fluid flow channels extending through the housing. A ball member is located within the cavity and is provided with an axial throughbore, which is selectively aligned with, or disposed transverse to, the fluid channels in the housing, by rotating the ball member about an axis of rotation to control the flow of fluid through the fluid channels. A pair of annular seats are located between the ball member and the internal wall of the housing and are positioned about the throughbore and the fluid channels to prevent fluids from leaking into the interior cavity of the valve.
In valve arrangements of the aforementioned type, seat life and fluid leakage has been a reoccurring problem. Since the ball member is constantly in sealing engagement with the seats, compressing them in both the open and closed valve positions, the seats tend to wear out after a period of time and need replacement. The problem is particularly manifested when the valve is used to control flow of an abrasive fluid, when the fluid has a relatively high pressure, and/or when the valve is used under service conditions which require that the valve be rapidly cycled between open and closed positions. The same problem is present to some degree in all types of ball valves in the course of fluid flow applications. When the seats have become worn, they are otherwise no longer capable of performing their intended sealing function and need replacement to eliminate consequent leakage of fluid between the housing and the ball member. Replacement of the seats requires that the valve be taken out of service and new seals or seats be installed.
In an effort to deal with the foregoing problems, valve arrangements have been designed that reduce seat loading when the valve is in its open position. For example, one ball valve design includes a split ball, wherein a cam, which rides within a split at the bottom of the ball, spreads the ball to form a tighter seal with the valve seats, as the ball is rotated to its closed position. Other designs utilize plugs or ball segments, which seal against a single seat in the housing, and which are mounted eccentrically on an actuator shaft or a stem, so that the plug is moved into forcible contact with the seat in the closed position of the valve. Moving the valve to the open position moves the plug away from the seat, allowing fluid to flow through the valve.
Valves employing the split ball design or eccentrically offset plugs are, however, relatively complicated and expensive to manufacture and maintain. Eccentrically mounted plugs also suffer from other disadvantages, since they involve an asymmetrical or unbalanced design. Specifically, eccentrically mounted plug valves are prone to leaking problems arising from rapid internal component wear, resulting from lack of structural support to counter forces created by high fluid pressures.
Therefore, there is a need for a fluid flow control valve that obviates all the above problems by providing a novel ball member having a symmetrical and balanced design, improving the internal structural support to counter forces created by high fluid pressures.
There is also a need for a ball member comprising an outer surface having a gradually increasing radius with respect to the axis of rotation. As the ball member rotates from the open valve position to the closed valve position, the outer surfaces gradually seal against a pair of associated upstream and downstream valve seats, to achieve maximum seal loading at the full closed valve position.
There is also a need for an improved ball member configured for use with conventional valve housing and seats, while improving valve life and sealing performance of the valve.